This invention relates to gas detecting apparatus, and more particularly, to natural gas detecting apparatus of the type which operate on the basis of absorption of infrared light.
Utilities which distribute natural gas require reliable, portable gas-leak detectors for use in maintenance of gas supply lines. Existing natural gas detectors are either costly, sensitive and non-selective or low cost, insensitive and non-selective. Non-selective gas detectors respond to any combustible gas. Selective gas detectors are specific to hydrocarbon gases. The two presently most used detectors are based on hydrogen flame ionization and on hot wire catalysis, cannot distinguish among different types of hydrocarbons. However, it is necessary to distinguish among different types of hydrocarbons in order to distinguish a pipeline gas from gasoline vapors or sewer or swamp gas and so reduce leak surveyor time wasted on false alarms. Ethane content, if measurable, provides a good means to discriminate between pipeline gas and interfering sewer or swamp gases because the latter contain practically no ethane, while pipeline gas does, in varying degrees. Gasoline vapors and propane (LP gas) can also generate a false alarm with conventional instruments. However, their infrared absorption is shifted relative to that of methane, as will be described later, as it is the basis for this invention to eliminate sending false alarms.
In U.S. Pat. No. 4,507,558, there is disclosed a selective detector for natural gas which discriminates between low concentrations of natural gas and other methane sources by measuring the characteristics of the methane/ethane ratio of natural gas as well as by using a combustible gas sensor. The operation of this detector is based on infrared light absorption of methane and ethane in combination with another non-specific combustible gas detector whereby the detector has the ability to detect non-specifically, the presence of a combustible gas, and to define the nature of the combustible gas. Thus, this natural gas detector utilizes two types of detection including nondispersive infrared detectors and a non-specific combustible detector such as hot-wire catalytic combustible detector. The detector determines concentration of both methane or ethane irrespective of the concentration of the other gas by using absorption cells placed in front of the detectors. The detector includes a light emitting diode which issues light centered around 3.32 microns and a reference light source which emits light at a wavelength outside of this band. Although this arrangement permits distinguishing among different types of hydrocarbons, the requirement for a hotwire catalytic combustible detector adds cost and complexity to the device and increases power consumption.
It would be desirable to have a natural gas detector which can distinguish among different types of hydrocarbons, and which provides information to the user on the amount and type of combustible gases in the environment.